1. Field of the Invention
The present invention relates to a tilt device for a marine propulsion unit.
2. Description of the Related Art
There exists a conventional tilt device for a marine propulsion unit in which a cylinder device is interposed between a stern bracket fixed to a boat body and a swivel bracket which supports a propulsion unit. An interior of the cylinder device is divided into a first tilt chamber closer to a side in which a piston rod is accommodated and a second tilt chamber closer to a side in which the piston rod is not accommodated by means of a piston. The tilt device includes a shock blow valve which makes it possible to expand the piston rod by sending oil in the first tilt chamber to the second tilt chamber through a communication hole formed in the piston which is opened and closed with a constant set pressure. In this prior art, when the marine propulsion unit collides against a floating log or the like, the shock blow valve is opened to turn up the propulsion unit, thereby absorbing the impact force.
In the prior art, the shock blow valve comprises a ball for sitting on a valve seat provided around the communication hole of the piston thereby closing the communication hole, and a spring for biasing the ball against the valve seat. The prior art has the following problems 1) to 3).
1) In the case of the shock blow valve using the ball, it takes time to completely close the communication hole after the hole is once opened, and the response is slow. This is because it takes time to obtain a sitting attitude of the ball, having critical clearance tolerances, with the result that the ball completely conforms to the valve seat with difficulty.
2) The setting of the valve opening pressure is difficult due to variation of the spring constant.
3) The ball and the spring occupy constant space in a thickness direction of the piston, and this fact increases the thickness of the piston. As a result, even if the cylinder's length is the same, a piston stroke in the cylinder is shortened.